Powertrains having engines connected to transmissions via torque converters are known. Torque converters are devices, typically hydrodynamic fluid couplings, that are used to transfer rotating power from a prime mover, such as an internal combustion engine, to a rotating driven load. For land vehicles, the driven load may be a driveshaft that is connected through a differential or other gear arrangements to one or more wheels or other driving mechanisms of the vehicle.
One characteristic of torque converters is their ability to multiply torque when there is a difference between the input speed to the converter from the engine and the output speed of the converter to the transmission. Some torque converters also include a locking mechanism that transfers engine speed directly to the transmission with no substantial torque multiplication or speed variation.
When the torque converter is used in conjunction with a multiple speed transmission, the locking state of the converter during different shifts can affect the operation of the engine in terms of fuel efficiency as well as in terms of rough engine operation, especially during shifts. For certain vehicle applications, such as wheel loaders and other heavy earth moving and construction equipment, fuel efficiency and smooth engine operation are desired. In such applications, certain of which may operate at a full throttle operating condition for prolonged periods, the engagement of the locking mechanism of the torque converter may be restricted to only a few gear ratio selections of the transmission that operate at engine and transmission speeds that are sufficiently similar to avoid power variation at the ground engaging members of the machine during gear shifts. However, such limitations on the gear selections that are possible for the machine may, overall, result in increased fuel consumption of the machine.
One example of a powertrain control that addresses at least some issues associated with improving fuel efficiency and engine roughness in heavy machinery can be found in U.S. Pat. No. 7,901,321 entitled “Automatic Transmission With Part-Throttle Shifting,” which issued on Mar. 8, 2011, to Ferrier et al. and is assigned on its face to Caterpillar, Inc., of Peoria, Ill. (“the '321 patent”), the contents of which are incorporated herein in their entirety by reference. Although the systems and methods disclosed in the '321 patent are useful in improving the fuel efficiency and operational smoothness of a machine, these benefits may not be realized for all gear ratios of the machine, thus not exploiting potential efficiencies. Further, engagement or disengagement of the lock-up clutch for all transmission gear ratios may cause an appreciable power variation to the wheels of the machine, especially for large machines that have engines with throttle lock controls, which means that operation of the engine is, at times, speed-control based. These and other issues may be addressed as follows.